Several severe health disorders require treatment of the brain. This includes neurodegenerative diseases such as Parkinson and Alzheimer diseases, but also central nervous system diseases such as schizophrenia, epilepsy or bipolar disorder, brain cancer, human immunodeficiency virus (HIV) and even certain aspects of obesity. The pharmaceutical targets of these diseases are located inside the brain.
The BBB is a natural filter within the body that regulates the passage substances through from the blood to the brain, allowing only certain substances to cross from the blood to the brain. It is a natural defense mechanism designed to keep harmful substances out of the brain. It controls the composition of the brain extracellular fluid independent of fluctuations within the blood. It is also impermeable for many environmental compounds and drugs.
The anatomical basis of the BBB is primarily the tight junction at endothelial cells of cerebral microvessels, which form a continuous membrane with no fenestrations. Specific transporters mediate the access of certain molecules important for the brain, such as glucose, isolated amino acids and ions. Other compounds can cross the BBB by a passive diffusion process through the endothelial cells that form the brain microvessels. However, this process requires a certain degree of lipophylicity of these compounds. This type of transport has the disadvantage of being very selective and to be regulated to some extent by efflux pumps, located in the membrane of endothelial cells that form the BBB, which prevent the accumulation of substances potentially toxic or unnecessary in the nervous system.
In the therapeutic areas mentioned above, many promising compounds are known for their treatment, however, owing to their BBB transport problems, they are not further developed. Research in these fields has taken several approaches.
Some methods of administration of drugs to the brain either for therapy or diagnosis are invasive techniques, such as intracranial administration, administration altering BBB integrity or osmotic disruption. However, these methods imply risk of infection and toxicity, and in addition, require qualified personnel.
Another approach is drug modification. These modifications include for instance reduction of drug size or increase of drug lipophylicity, but it is not always possible to introduce such modifications. In the case of introducing an irreversible modification it is necessary that it doesn't alter the drug activity once it gets to the target site. In the case of a bioreversible modification, it is necessary to find an enzymatic or chemical process that will recover the active drug once the prodrug is inside the central nervous system.
Another approach is the administration of the drug by conjugation to a biological carrier. This strategy is based on the conjugation of the drug to a substrate of a specific protein receptor or to an antibody that recognizes selectively a given receptor, e.g. the transferrin receptor or the receptor of LDL. This drug-carrier conjugate can be recognized by a particular receptor and undergoes a receptor-mediated endocytosis process into the nervous system, in such a way that compounds with a potential therapeutic use which cannot cross the BBB by themselves became able to cross this barrier when are conjugated with the carrier.
Finally, another approach comprises a peptide vector-mediated strategy in which non-transportable drugs are linked to peptides that have the capacity to cross the BBB (cf. J. Temsamani et al., “Improved Brain Uptake and Pharmacological Activity Profile of Morpholine-6-Glucuronide Using a Peptide Vector-Mediated Strategy” in J. Pharmacol. Exp. Ther. 2005 vol. 313, pp. 712-719). This approach it has been use to improve the brain uptake of several drugs, such as doxorubicin, penicillin, enkephalin analog dalargin, paclitaxel and morphine-6-glucuronide. In some cases, this conjugation even increased drug solubility and bypassed the P-glycoprotein expressed in the BBB.
WO2008025867 describes several compounds based on the structure of the diketopiperazine that act as vehicles for delivery active pharmaceutical ingredients through the BBB due to they have the capacity to carry drugs that lack capacity to pass the BBB, into the brain by passive diffusion. The permeability of the construct cargo-shuttle through the BBB was studied by means of the PAMPA assay, an in vitro model of the BBB.
Likewise, WO2012007625 describes the use of phenyl proline (PhPro) rich peptides as shuttles of compounds that are unable to cross the BBB. The described peptides can facilitate the transport of these compounds into the central nervous system. This patent application, as WO2008025867, is based in the use of shuttles that are able to cross the BBB by a passive diffusion mechanism. This fact limits their use as shuttles to the transport of small molecules.
Several peptides have been described whose mechanism of transport across the BBB is through an active transport process of receptor-mediated transcytosis.
WO200244329 describes a peptide, THRPPMWSPVWP (SEQ ID NO:5) (L-amino acids), which have the capacity to bind to the human transferrin receptor (hTfR) and internalized into cells expressing hTfR. This peptide can be link to other molecules, such as other peptides or proteins, in order to facilitate the transport of these molecules into cells expressing hTfR. The peptide and its ability to bind hTfR also described in Lee et al., in Eur J. Biochem. 2001, vol. 268, p. 2004-2012.
WO2007098415 describes that the peptide THRPPMWSPVWP (SEQ ID NO:5) (L-amino acids) can be used in vivo in order to direct macromolecules into cells expressing hTfR. In particular it is indicated that enhances release of peptide derivatives that inhibit the interaction of proliferating cell nuclear antigen (PCNA) in a specific manner and thus reduce cellular proliferation of malignant cells that express an isoform cancer specific caPCNA. Unfortunately, the peptide THRPPMWSPVWP (SEQ ID NO:5) (L-amino acids) shows a low half-life time in human serum, thus, limiting their applications.
Finally, WO2009008725 describes peptides or peptidomimetics comprising the sequence THRPPMWSPVWP (SEQ ID NO:5) (L-amino acids), as well as the conjugates that comprise these peptides and biological molecules or diagnostic, and their use to direct diagnostic biological molecules across the blood-brain barrier, brain cells, neuronal cells or tumor cells of neuroectodermal origin or neurons for the treatment of related disorders. The peptidomimetics described refers to variants of the above peptide comprising an amino acid substitution in the sequence for another amino acid or derivative of this one. Also describes that the peptide may consist entirely of L-amino acids or may contain one or more modifications in the backbone or side chains. All examples of the application refer to L-amino acids.
On basis of the existing results, it would be desirable to have a compound with the capacity to cross the BBB able to transport cargos across it in an efficient manner. Likewise, it would be interesting that these compounds had a high stability in physiological conditions in order to avoid side effects and improve its efficiency.